In the current Long Term Evolution (LTE) design for high-speed wireless communication, a transport block (TB) is the basic transmission unit for the physical (PHY) layer. A TB typically includes the transmission data payload and parity bits for error detection and/or error correction, such as cyclic redundancy check bits. TBs can be divided into several Forward Error Correction (FEC) code blocks, which are also commonly referred to as “code blocks”.
Hybrid-automatic repeat request (H-ARQ) is a retransmission scheme typically based on a combination of high-rate forward error-correcting coding and error-control. In LTE, to save on feedback overhead, H-ARQ retransmission is TB based. An outer code is applied to a set of code blocks to produce outer code information bits, and outer code parity bits are determined based on the outer code information bits.
In conventional applications, the outer codes are essentially erasure codes. The receiver attempts to decode each of the code blocks. If decoding of one or more code blocks fails, the receiver requests a retransmission, possibly specifying how many code blocks have failed, but not necessarily identifying the failed code blocks. Because the transmitter does not know which code blocks failed, the retransmission contains outer code blocks that include redundancy bits for all code blocks, even those that were successfully decoded. The receiver then uses the outer code blocks to recover the failed code blocks.
Examples of suitable outer codes include parity-check codes and Reed-Solomon codes. Depending on the code, some number of failed code blocks can be decoded. When then number of failed code blocks is too high, H-ARQ is used to trigger a retransmission for the TB. These codes are more effective when the number of failed code blocks is small.
There is a desire to have more efficient or more flexible retransmission protocols to correct transmission errors, and to save bandwidth from retransmissions that could be used for other data.